Mass transfer processing of aqueous solutions containing ionic components, for the purpose of separation of the substances contained in them, concentration of solutions by separate components or a group of components, extraction of individual components or refinement of solutions from individual components or a group of components are one of the most widespread, basic processes in today's chemical productions and technologies.
There are a lot of standard methods for the mass transfer sorption processes of component separation of aqueous solutions of inorganic substances, with application of a variety of sorbents, including ion-exchange materials (e.g. see: Senyavin M. M. “Ion exchange in the technology and analyses of inorganic substances”, Moscow, “Chemistry” ed., 1980, p. 272 [1]). These methods include periodic passage of the processed solution through a layer of granular ion exchange material in the column, e.g. through a layer of ion exchange resin, previously transformed into some ionic form. The ion exchange process and to the difference in selectivity of ion exchange resins to various components result in transition of the latter either to solid or to liquid phase, i.e. the processed solution is refined from some components and enriched with others. These methods have the following main limitations: they require application of chemical agents for regeneration of ion exchange resins and re-using of them in the cycles of sorption-desorption. In this regard, standard ion-exchange processes are applied only for the processing of diluted solutions. Processing of concentrated solutions, which are characterized by the so-called “short” sorption cycles, becomes unprofitable because of excess consumption of reagents for regeneration per unit volume of the processed solution. In addition, in processing of solutions in short cycles, longitudinal mixing takes place due to differences in liquid densities in the system, namely of the solution, supplied in the ion-exchange column and the solution in the space between granules in the ion exchange resins' layer, i.e. in the space corresponding to the porosity of the layer. It significantly reduces the efficiency of processing and results in large amounts of waste water in the form of mixed solutions. Particularly, it concerns the so-called once-through ion exchange systems, in which the processed and regeneration solutions are transmitted through mass-exchange columns in the same top-down direction.
Reagent's flow rate and the volumes of waste water are reduced by using the so-called counter-flow patterns, in which regeneration solutions of high density are transmitted through columns bottom-upwards. However, even in this case, the effects of longitudinal mixing are difficult to eliminate because of formation of “flow channels” in the layers of sorption materials, which is caused by the effects of granule compression in ion-exchange materials in concentrated electrolyte solutions [1].
There are also reagentless methods for the mass transfer sorption processes of component separation of aqueous solutions of inorganic substances with the use of ion exchange resins with temperature-controlled selectivity (B. A. Bolto, D. E. Weiss. In Ion Exchange and Solvent Extraction (Eds. J. A. Marinsky and Y. Marcus). Marsel Dekker, New York, 1977. P. 221 [2]; Russian Federation Patent No 2034651, publ. May 10, 1995 [3]. According to these methods, regeneration of ion exchange resins in cyclic processes is performed with hot water, or processed solution, or obtained intermediate solutions at temperatures, different from the temperatures of sorption processes. The indicated methods are also intended for the processing of diluted solutions. Critical degrees of enrichment or purification of solutions, achieved by using them, are limited, among other factors, by free spaces between granules in the sorbent layer. Other limitations of these methods are the small range of applicable thermo-selective ion exchange resins and the increased energy costs, caused by the necessity to heat and (or) chill solutions and ion exchange materials.
In addition, there is a known method for the mass transfer sorption processes of component separation of concentrated aqueous solutions of inorganic substances, in particular, of concentrated mixed solutions of salts and acids with common anion; namely the method of separation of acids from salts with application of ion exchange resins, which is called the method of “acid retardation” (M. J. Hatch, J. A. Dillon. Industrial and Engineering Chemistry Process Design and Development, 1963, V. 2, No 2, p. 253 [4]). There is no ion exchange in the indicated method. Separation of cations is performed on anion exchange resin, rather than on cation exchange resin, taken in the form of anion, cognominal with electrolytes. For example, sulfuric acid is extracted from the mixture of sulfate salts by transmission of mixed solution through ion exchange resin in the sulfate form; in cases of reprocessing of a mixture of nitrates ion exchange resin is used in the nitrate form; and in the cases of chloride reprocessing—in the chloride form. The method is based on the fact that, in concentrated mixed solutions with total concentration of components from 3 to 20 g-ppm, water activity is low and the dissociation degree of salts and, particularly, of acids is significantly lower. Tightly connected ion pairs as well as molecules are formed in the solutions and are capable of non-exchangeable sorption in ion exchange resin. Separation of components takes place during passage of mixed concentrated solutions through a layer of ion exchange resin in the column: at first, the salts of multivalent cations come out of the column; after that, the salts of monovalent cations come out with a small delay; and finally acid solution comes out with considerable delay, comparable by the volume of the treated solution with the volume of ion exchange resin. Once the balance is achieved, i.e. enrichment of ion exchange resin, the compositions of the solutions at the input and output of the column become identical. Separation is carried out as follows: a solution passes through the column until the acid solution is formed; the latter is desorbed along with a small admixture of sorbed salts with the help of water, used as eluent; after that, the column is ready for the separation of the next portion of processed electrolyte solution.
The above mentioned method [4] is almost analogous to those, which involves passage of electrolyte solutions through ion exchange resin or other sorption materials, using water or one of the components of separated mixture as eluent (Patent of Russian Federation No 2056899, publ. Mar. 27, 1996 [5]). In this method, small effects are achieved in the separation of salts, along with the separation of acids from salts.
The main limitations of the methods [4] and [5] are that using standard ion-exchanging apparatus and straight flow during solution passage through the layers of sorption materials, mostly results in mixing of solutions due to the effects of longitudinal mixing of solutions having different densities in the scope of porosity. In case of the counter-flow processes, when concentrated electrolyte solution is passed through the column bottom-up, and water—top-down, use of the majority of well-known ion exchanging resin—(cations and anion exchange resins) also results in large amounts of mixed solutions, caused by lower selectivity of sorption electrolytes in ion exchange resins in comparison with water sorption, and also by the effect of channel formation in the layer of granulated sorption material, due to the compression of granules of ion exchanging materials. In these methods [4, 5], the processed solutions rather than the solutions for regeneration are the chemical agents, lost due to the formation of mixed solutions, or threatening the environment. Nowadays, processes of dilution and leaching are applied in many fields of chemical industries, hydrometallurgy, ferrous and nonferrous metallurgy, electro-planting industry and other fields, resulting in the formation of either expensive or environmentally harmful concentrated solutions, which require reprocessing. In particular, this concerns separation of concentrated solutions of acids and salts.
There is another method of mass transfer sorption processes of component separation of aqueous solutions of inorganic substances (Khamizov R. Kh., Myasoedov B. F., Rudenko B. A., Tikhonov N. A. Reports of the Academy of Science, 1997, Vol. 356, No 2, p.p. 216-218 [6]; D. N. Muraviev, R. Kh. Khamizov, N. A. Tikhonov, V. V. Kirshin. Langmuir, 1997, V. 13, No. 26, p.p. 7186-7192 [7]). This method allows reducing consumption of chemical agents to the minimum amounts, equivalent to the quantities of output products; as well as preventing formation of hardly recyclable mixed solutions. A phenomenon of isothermal supersaturation of solutions in ion exchanging processes is applied in this method, as well as stabilization of supersaturated solutions in the layer of ion exchange resin. The ion exchanging material, used as a sorbent is previously transformed into ionic form, e.g. in the form of metal ion; after that it is treated with concentrated solution of a substance, containing anion, that forms a compound with metal ion, less soluble, than a substance for treatment. For example, to obtain magnesium carbonate, magnesium form of cation exchange resin is treated with concentrated solution of sodium or ammonium carbonate; and to obtain potassium sulfate, potassium form of cation exchange resin is treated with concentrated solution of sodium or ammonium sulfate. In the result of ion exchanging process, a supersaturated solution of target compound is formed in the layer of ion exchange resin and stabilized for some time. At the output of supersaturated solution from the column, clear end product, separated from the solution, is spontaneously crystallized.
The latter is additionally reinforced by regenerating substance and directed into the next cycle of ion exchange resin treatment in the required ionic form. These methods, irrespective of longitudinal mixture effects, do not cause loss of agents or formation of mixed waste water, which require additional treatment. However, the drawbacks of these methods [6, 7] consist in the facts that stabilization of supersaturated solutions in the ion exchange resin layer is temporary, and for many components stabilization period is insignificant. On the one hand, this reduces the range of applied ion-exchanging systems; on the other hand, it causes a risk of column hardening, i.e. sedimentation in the space between granules of the sorbent, in the scope of porosity.
The closest method to the proposed one is the mass transfer sorption process of component separation of aqueous solutions of inorganic substances under the U.S. Pat. No. 4,673,507 (publ. Jun. 16, 1987) [8]. According to this method, aqueous solutions are processed with the use of granular sorption material layers, with reduced spaces available for the processed solution between sorbent granules. Solution is reprocessed according to the indicated method by transmission of it through highly compressed short layer of finely divided granular sorption material. The layer is compressed in different ways: e.g. when working with diluted solutions, a granular ion-exchanging material in the media of highly concentrated electrolyte is loaded in the apparatus, so that the sorbents swell in the media of operating solutions. Another method is loading of redundant amount of sorption material into the open apparatus (without cover), which is forcefully formed into highly compressed layer under the pressure of a cover of special design. Due to high pressure, required for pumping of processed solution through this apparatus, in the method under patent [8], as a rule, a short sorption layer is used. A finely divided sorbent with good kinetic performance is used to reduce the front line length of ion-exchanging or molecular sorption that is required in the use of short layers.
The mentioned requirements show, that there are some limitations in the implementation of the method under patent [8]. Drawbacks of this method include the necessity to apply high pressure. In addition, drawbacks include the short life of sorbents, when using them in the cycles of sorption-regeneration. Finally, in the implementation of the mentioned method, in comparison with standard methods, the increase of stability of supersaturated solutions in the sorbent layer cannot be achieved, which reduces the range of processed solution.
There are apparatus for the implementation of mass transfer sorption processes that are being improved to enhance their efficiency.
Thus, the apparatus under the USSR Inventor's Certificate No 1183146 (publ. Oct. 7, 1985) [9] contains vertical cylindrical body with bottom and cover, designed for filling with sorbent, which has inlet and outlet brunch tubes, placed respectively in the bottom and cover, as well as distribution system, designed in a special way and connected with the inlet brunch tube. This system includes a package of stacked canvases with tunnel cells, which edges on each next canvas are shifted in relation to previous canvas in the horizontal plane in the same direction. This provides torsion of the processed solution and more uniform and intensive interaction of it with the sorbent. However, the described implementation of the distributional sys-tem causes high resistance to the flow of the processed solution, so it will be supplied under increased pressure.
An apparatus under the USSR Inventor's Certificate No 1533750 (publ. Jan. 7, 1990) [10] contains vertical housing with cover, bottom and brunch tubes for input and output of solution, and also a cylindrical microcellular drainage system with installed piston, coaxially placed in the casing. The space between drainage system and housing walls shall be filled with sorbent. A piston performs reciprocating movement that intensifies both absorption of purified solution through the drainage system, and cleaning of its surface from sticky sorbent. The drawback of this apparatus is the need for mechanical drive.
The closest apparatus to the proposed one, designed for the implementation of mass transfer sorption processes by the proposed method, is the apparatus known under the U.S. Pat. No. 4,673,507 [8], designed for the implementation of the method, described in this patent.
This apparatus has a cylindrical body with bottom, cover, wall, upper and lower brunch tubes, installed respectively into cover and bottom. The upper and the lower drainage-distributional systems are installed in the housing, hydraulically connected with upper and lower brunch tubes respectively. The space between these systems shall be filled with granular sorption material. The latter forms a highly compressed layer. The apparatus is operated with the use of finely divided sorbent with good kinetic parameters.
Drawbacks of this apparatus correlate with the above mentioned drawbacks of the method under patent [8], for which this apparatus is intended. In particular, this is the necessity to generate and maintain high pressure during exploitation of the apparatus. This causes fragility of sorbents, when they are used in the cycles of sorption-regeneration. High compression of sorption layer results in high hydraulic resistance, that's why the layer will be short. As a consequence, stability of supersaturated solutions, achieved in the sorbent layer is low and the range of processed solutions is limited.
There are plants of the same function, in particular, under the Russian Federation Patent No 2034651 (publ. May 10, 1995) [3] and under the U.S. Pat. No. 4,673,507 (publ. Jun. 16, 1987) [8].
In the unit under patent [3] a reagentless method is used for the implementation of mass transfer sorption processes of component separation of aqueous solutions of inorganic substances. This unit contains ion-exchanging columns interconnected in parallels, heat-exchanger, several pumps, a selection line of end product and a system of electronic control, thermostat heaters of initial solution and concentrate, a system of pipelines and a range of other parts.
Due to free spaces between sorbent granules in this unit, the achieved degree of enrichment is low. The unit allows processing of only diluted solutions, it has a complicated structure and requires high power consumption, because of thermostat heaters.
The closest unit to the proposed one is the unit under patent [8]. This unit contains an apparatus for the implementation of mass transfer sorption processes and a means of pumping liquid through it. The indicated apparatus has a cylindrical housing with bottom, cover, wall, upper and lower brunch tubes, installed correspondingly in the cover and bottom. Upper and lower drainage-distributional systems are installed in the housing and hydraulically connected, correspondingly with upper and lower brunch tubes. The space between these systems is intended for filling with granular sorption material. The latter forms a highly compressed layer. The apparatus is operated with the use of fine sorbent with good kinetic parameters.
In this plant, free spaces between sorbent granules in the mass transfer sorption apparatus are reduced, that contributes to enhancing of the efficiency of mass transfer sorption processes of component separation of aqueous solutions. However, the indicated reduction of free spaces is achieved by high compression of sorption layer, causing increase of its hydraulic resistance and conditioning the necessity to use a means of generation of high pressure for pumping of liquids. High compression of sorbent layer causes fragility of the sorbent. Besides, the highly compressed layer shall be short, that results in low stability of supersaturated solutions in the sorbent layer and reduces the range of processed solutions.